And this interdependence had to include all 10,000 in the cluster. The studio couldn't hand-animate the balloons, nor would traditional computer animation work. Instead, Pixar's animation and computer experts crafted a procedural animation technique that let them achieve the look they wanted.

Progression 1 of 8

In this, the first of eight evolving versions of an animated scene from Pixar's "Up," we see early drawings depicting the film's two main characters--Carl and an eight-year-old stowaway named Russell--as they try to navigate a river.

Progression 7 of 8

Progression 8 of 8

Finally, all the elements of the scene are pulled together into the final version. We see ambient light added, as well as much more realistic shading and the characters' animation details. This is how the scene looks in the film.

Interdependent balloons

"Up" revolves around the idea of using a cluster of more than 10,000 balloons to hoist a house aloft and carry it away for global adventuring. But animating the balloons was a major computing challenge, and here, with the cluster seen up close, we can see why: Each balloon has its own string, its own color, its own reflection of light, and each balloon is interdependent.

Carl's face

Animating the film's main character, 78-year-old Carl Fredricksen, was a challenge, too. His face, designed with a cube-like shape to represent his lifelong feeling of being boxed in, required the animators to figure out how to make his facial expressions seem real.

Feathers

Another challenge was animating the feathers on Kevin, the bird character in "Up." While Pixar has mastered animating flowing fur ("Monsters, Inc.") and underwater imagery ("Finding Nemo"), it had to figure out how to use procedural animation for something as interdependent as a bird's feathers.

Balloon house

In this image from "Up," we see the balloon cluster carrying the house over a gorgeous canyon. This image allows us to see the full size of the balloon cluster, which was comprised of about 10,000 balloons. However, in real life, it would take more than a million balloons to hoist a house.